added READONLY option to password wallet

Made an update to my password wallet. You can now have the READONLY attribute in your .walletrc file: this disables updates to the wallet (w/ the -e option). I keep my wallet in two places (work and home), and a cron job copies from work to home daily. So I need to make sure that I only update the wallet at work. I once updated it at home, and the next run of that cron job overwrote the update (a new password).